Cell dependent multi-group hybrid automatic repeat method for multicast in wireless networks

ABSTRACT

A method and apparatus are described including determining address using an access point address and a multicast group address, transmitting a recovery request message to a recovery server to request recovery data using the address and receiving the recovery data from the recovery server. Also described are a method and apparatus including receiving a registration message, transmitting a reply to the registration message, receiving a recovery request message, transmitting recovery data responsive to the recovery request message and transmitting a message to a recovery multicast group to determine status of the recovery multicast group.

This application claims priority to European patent application SerialNo. EP 08305230.8, filed on Jun. 4, 2008 and entitled “STREAMING DATAPAUSE FUNCTIONS”, incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to wireless communications and, inparticular, to a method for cell-dependent retransmission of data.

BACKGROUND OF THE INVENTION

Interest in and demand for wireless multimedia applications are growingrapidly. Multicast over wireless networks enables efficient distributionof data including but not limited to multimedia data and entertainmentprograms (video, audio, file and text) to many receivers simultaneously.However, multimedia data delivery requires high reliability andbandwidth efficiency. Wireless links are unreliable with time-varyingand burst link errors. Especially in multicast applications, differentreceivers of the same program may experience heterogeneous channelconditions and receivers may leave or join during the session so thatthe topology of receivers changes. There is no link layer retransmissionand no link layer adaptation for multicast in many wireless networks,for example, current IEEE 802.11 networks. A wireless device may behanded off from one access point

(AP)/base station (BS) to another. As used herein, “/” denotesalternative names for the same or similar components or structures. Thatis, a “/” can be taken as meaning “or” as used herein. A wireless deviceincludes laptops, dual mode smart phones, personal digital assistants(PDAs), end devices, clients, client devices, mobile devices mobileterminals, multicast clients, multicast client devices, receivers etc.Data frame transmission is interrupted and a number of data packets arelost to the receiver during periods of handoff. Furthermore, multiplewireless APs/BSs are connected to a multicast server/source via a highspeed wired network. The channel conditions of the clients in one cellmay be dramatically different from the channel conditions of clients inanother cell.

A unicast communication is a one-to-one communication between twoentities. A broadcast communication is a communication between one andall other entities in the communication system. A multicastcommunication is a one-to-many communication between an entity and aplurality of other entities in the communication system, where theplurality of other entities may include all other entities in thecommunications system. In many wireless multicast systems, the forwarderror correction codes (FEC) are used within a packet at the physicallayer to protect against multi-path fading and interference and toreduce the packet errors. To recover the lost packets in wirelessnetworks, the FEC codes are also applied across packets at the transportand application layers. However, the wireless channel conditions aretime-varying and the multiple clients in multicast networks experienceheterogeneous channel conditions. The FEC codes are normally usedaccording to the worst channel conditions to ensure the receivingquality of all the receivers in the desired service area. This resultsin a large overhead and requires a great deal of radio resources forretransmission. Another technique to improve reliability is to useretransmission of lost packets, also called Automatic Repeat reQuest(ARQ). In a multicast scenario, ARQ is not very efficient. For example,if client A loses packet 1 and client B loses packet 2. Thesource/server retransmits both packet 1 and packet 2.

In some reported multicast systems, a hybrid ARQ scheme is described,which combines ARQ and FEC and is more efficient than pure ARQ. Theretransmitted packets in hybrid ARQ are the parity packets generated bya FEC code, which can be used to correct different lost informationpackets by different receivers. For example, client A loses packet 1 andclient B loses packet 2 and if a Reed-Solomon FEC code is used togenerate the parity packets with cross-packet erasure coding, thesource/server only needs to retransmit/multicast one FEC parity packet.Client 1 can use the retransmitted FEC parity packet to recover packet 1and Client B can use the same FEC parity packet to recover packet 2.

However, in the previous multicast ARQ or hybrid ARQ schemes, theretransmitted packets are sent in a single multicast group, either thesame multicast group as the source data packets or a separate multicastgroup.

Multiple receivers may experience time-varying and heterogeneous channelconditions. If one receiver in a particular cell requests a lot ofretransmitted packets, these retransmitted packets will be sentmulticast in all the wireless cells even though the receivers in othercells do not need so many retransmitted packets. This method, thus,wastes the radio bandwidth of the other cells.

Scalability in terms of the number of APs/BSs and the number of clientsis an important issue to resolve. It would be advantageous to have areliable and scalable wireless multicast system and that efficientlyutilizes the wireless bandwidth.

SUMMARY OF THE INVENTION

Described herein is a multi-group hybrid Automatic Repeat reQuest (ARQ)(also called recovery herein) method for reliable multicast overinfrastructure/cellular wireless networks. The hybrid ARQ method of thepresent invention automatically assigns a multicast group to thewireless devices associated with an access point (AP)/base station (BS)or in a wireless cell for cell-dependent control message exchangesincluding, for example, sending the ARQ request and receivingretransmitted packets. The control messages and retransmitted data forthe multicast clients in one cell will not be transmitted by otherAPs/BSs over the wireless links in other cells. In this way, thewireless bandwidth used for retransmission in one cell is adapted to thechannel conditions of the multicast clients in this cell and is notimpacted by the channel conditions of the multicast clients in othercells.

A method to automatically assign a multicast address to a cell forcell-dependent control message exchanges and ARQ/retransmission, calledARQ multicast group, and to advise the ARQ/recovery server of theassigned multicast address of the ARQ multicast group is also described.Furthermore, the present invention describes the method that the ARQserver joins a cell-dependent ARQ multicast group for a cell when theARQ multicast group is needed and leaves the ARQ multicast group afterthe ARQ multicast group is terminated.

A method and apparatus are described including associating with anaccess point, obtaining an address of the access point, obtaining amulticast group address and determining a recovery multicast addressusing the access point address and the multicast group address. Alsodescribed are a method and apparatus including transmitting a request tojoin a multicast group, transmitting a request to join a recoverymulticast group, transmitting a registration message to a recoveryserver, receiving multicast data, determining if any data has been lost,determining if another device has transmitted a recovery requestmessage, generating the recovery request message, if no other device hastransmitted the recovery request message, determining if a registrationreply message has been received from the recovery server, determining ifany retransmitted data has been received previously and unicasting therecovery request message to the recovery server if no retransmitted datahas been previously received. Further described are a method andapparatus including receiving a recovery multicast group message from arecovery server, determining if a device is a member of a recoverymulticast group, transmitting a reply to the recovery multicast groupmessage.

A method and apparatus are described including determining address usingan access point address and a multicast group address, transmitting arecovery request message to a recovery server to request recovery datausing the address and receiving the recovery data from the recoveryserver. Also described are a method and apparatus including receiving aregistration message, transmitting a reply to the registration message,receiving a recovery request message, transmitting recovery dataresponsive to the recovery request message and transmitting a message toa recovery multicast group to determine status of the recovery multicastgroup.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is best understood from the following detaileddescription when read in conjunction with the accompanying drawings. Thedrawings include the following figures briefly described below wherelike-numbers on the figures represent similar elements:

FIG. 1 is a schematic diagram of an exemplary multicast system such asused in accordance with and to describe the method of the presentinvention.

FIG. 2 is an example of FEC coding.

FIG. 3 is a flowchart depicting an exemplary method in accordance withthe principles of the present invention for a client to obtain the ARQmulticast address for a program X.

FIG. 4 is a flowchart illustrating an exemplary method in accordancewith the principles of the present invention for a client to registerwith an ARQ server and send the ARQ server a request to receive packets(FEC and/or source) from the ARQ server.

FIG. 5 is a flowchart depicting an exemplary method in accordance withthe principles of the present invention for an ARQ server to process aregistration message received from a client.

FIG. 6 is a flowchart illustrating an exemplary method in accordancewith the principles of the present invention for an ARQ server toprocess an ARQ request message received from a client.

FIG. 7 is a flowchart depicting an exemplary method in accordance withthe principles of the present invention for an ARQ server to query anARQ multicast group.

FIG. 8 is a flowchart illustrating an exemplary method in accordancewith the principles of the present invention for a client to process anARQ multicast group query from the ARQ server.

FIG. 9 is a block diagram of an exemplary client/receiving device inaccordance with the principles of the present invention.

FIG. 10 is a block diagram of an exemplary recovery server/ARQ server inaccordance with the principles of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of an exemplary multicast system such asused in accordance with and to describe the method of the presentinvention. The wireless devices/clients/receivers 105 are connected to amulticast server 120 (e.g. a video streaming server) through wirelessaccess points (AP)/base stations (BS) 110 and a high-speed wired accessnetwork 130 (e.g. Ethernet) having an Ethernet switch/router 115. Anaccess point 110 serves a cell. There may be many multicastreceivers/clients 105 in a cell. The multicast server/video streamingserver 120 is connected to multiple APs/BSes 110 via the high-speedwired network 130 and serves multiple cells to increase the servicearea. To reduce the interference, the adjacent wireless access pointsmay operate using different frequency carriers. The server 120 sendsmulticast source data packets (e.g. compressed video data packets) tomultiple APs/BSes 110 and each AP/BS 110 transmits the source datapackets to a number of the clients 105 in its cell in multicast.

The ARQ/hybrid ARQ server 125 is also connected to the high-speedEthernet LANs 130 through Ethernet switch/router 115. The ARQ serverobtains the source packets from the multicast server via Ehernetswitch/router 115. The ARQ server 125 includes, among others components,a FEC encoder and an ARQ handler. The FEC encoder applies cross-packetFEC coding to the source packets and generates the FEC parity packets.The ARQ handler is responsible for retransmission of FEC parity packetsor source packets based on the request from the receivers. The FEC usedcan be any systematic forward correction code, for example, Reed-Solomon(RS) codes. The FEC code is used across the packets to protect againstloss (erasure) of an entire source packet because the erroneous packetsare often discarded by the lower layer of the protocol. For example, asshown in FIG. 2, a (N, K) RS code is applied to K source/informationpackets to form H=(N-K) parity packets. Each parity packet can recoverone lost source packet at the receiver no matter which source packet islost out of the K information packets. In an alternative embodiment, theARQ server 125 of FIG. 1 is collocated with the multicast server 120 ofFIG. 1.

In multicast, multiple receivers of the same video stream may experiencedifferent packet loss rates due to different channel conditions at thesame time. The same receiver may also experience different packet lossrates at different times. New receivers may join during the session orsome receivers may leave so that the topology of the receivers changes.For a multicast program, the channel conditions of the clients in onecell may be significantly different from that of the clients in anothercell. The hybrid ARQ method of the present invention is a method forreliable multicast over infrastructure/cellular wireless networks. Thehybrid ARQ method of the present invention automatically assigns amulticast group and multicast group address to the wireless devicesassociated with an access point (AP)/base station (BS) or in a wirelesscell for cell-dependent control message exchanges, such as sending theARQ request and receiving retransmitted FEC parity packets and/or sourcedata packets. The retransmitted data for the receivers/clients in onecell will not be transmitted by other APs/BSs over the wireless links inother cells. In this way, the wireless bandwidth used for retransmissionin one cell is adapted to the channel conditions of the multicastclients in this cell and is not impacted by the channel conditions ofthe multicast clients in other cells.

In the present invention, a method to automatically assign the multicastaddress to a cell for cell-dependent control message exchange andARQ/retransmission, called ARQ multicast group, and to advise the ARQserver of the assigned multicast address of the ARQ multicast group isalso described. Furthermore, the present invention describes the methodfor the ARQ server to join a cell-dependent ARQ multicast group for acell when the ARQ multicast group is needed and to leave the ARQmulticast group after the ARQ multicast group is terminated.

The method and system described herein can be used in multicastapplications over wireless local area networks (WLAN), 3G networks,WiMax, or other wireless networks although the IEEE 802.11 WLAN networksare used as an example to describe the adaptive FEC method and system ofthe present invention. Furthermore, the present invention is independentof the type of data that is being transmitted, and can be used formulticast of any type of data, and is not limited to audio/videoprograms although video multicast is used as an example to describe themethod of the present invention. The present invention can be used toretransmit the FEC parity packets and/or source packets.

For reliable multicast, the server transmits multicast source datapackets of a program to the clients in a multicast group, for examplemulticast group X. A client that desires to receive the source datajoins/subscribes to multicast group X by sending a request to the BS/APwith which the client is associated. The BS/AP transmits the data formulticast group X over wireless links in its cell as long as any clientin the cell is a member of multicast group X. If no client associatedwith a BS/AP wants the data for a particular multicast group, i.e. wantsto be a member of a particular multicast group and receive the datadestined for that multicast group, the BS/AP will not transmit the datafor that particular multicast group in its wireless cell, but discardsthe data. An BS/AP may periodically query the clients associated with itwhether any client is still a member of a particular multicast group.The Internet

Multicast Management Protocol (IGMP) or other protocols can be used as ameans by which the client can join or leave a particular multicast groupvia a request to the BS/AP. An alternative embodiment is that the clientcan sends a request to the Ethernet switch/router indicating that theclient wishes to join or leave a multicast group. If no clientassociated with a BS/AP wants the data for a particular multicast group,the Ethernet switch/router will not transmit the data for that multicastgroup to the BS/AP.

In addition, the clients of multicast program X in cell 1 join amulticast group 1X to send the ARQ request to the ARQ/hybrid ARQ serverand receive the retransmitted FEC parity packets or source packets fromthe ARQ/hybrid ARQ server in order to recover lost packets of multicastprogram X. In general, the clients of multicast program X in cell N joina multicast group NX to send the ARQ request to the ARQ/hybrid ARQserver and receive the retransmitted FEC parity packets or sourcepackets from the ARQ/hybrid ARQ server to recover lost packets ofmulticast program X. Multicast groups 1X, 2X . . . NX . . . aredifferent multicast groups with different multicast addresses. The ARQserver sends retransmitted FEC parity or source packets to multicastgroup 1X for the clients in cell 1, 2X for the clients in cell 2, . . .NX for the clients in cell N. All the source packets and retransmittedpackets are transmitted to the Ethernet switch/router over thehigh-speed wired network. Source data in multicast group X, as well asthe ARQ request and retransmited data in multicast group 1X, istransmitted in cell 1 because the clients in cell 1 are members ofmulticast groups X and 1X. However, the ARQ request and retransmitteddata for other groups (2X, . . . , NX, . . . ) is not transmitted incell 1 because clients in cell 1 are not members of multicast groups 2X. . . , NX, . . . . The data for other groups are discarded by cell 1,AP/BS or the Ethernet switch/router as described above. In this way, thewireless bandwidth is adapted to the multicast clients in cell 1.Similarly, the source data in multicast group X as well as the ARQrequest and retransmited data in multicast group NX is transmitted incell N. The ARQ request and retransmitted data in other groups (1X, . .. , (N−1)X, . . . (N+1)X, . . . ) is not transmitted in cell N.Therefore, the wireless bandwidth used for retransmission in a cell onlydepends on the channel conditions of the clients in that cell, not thechannel conditions of the clients in other cells.

The multicast addresses for the multicast source data as well as the ARQrequest and retransmited data need to be known by the clients in a cell,the multicast server and the ARQ server. The present invention furtherdescribes a method to assign the multicast addresses. A 32 bit IP v4address is used as an example to explain the address assignment methodof the present invention. The method of the present invention can easilybe extended to the assignment of a 128 bit IP v6 address or layer 2 MACaddress.

For a multicast program X, a 32-bit IP v4 multicast address bx(31),bx(30), . . . bx(m+1), bx(m), bx(m−1) . . . bx0, (bx(m) is the mth bitof the address, 0<m<31), is assigned for source data transmission,called source data multicast group, in which bx(31) . . . bx(m+1), bx(m)are either 1 or 0, and bx(m−1), . . . b0 are equal to 0. The source dataaddress for multicast program X can be configured at the multicastserver, the ARQ server and the clients. In an alternative method, thesource data multicast group address for multicast program X can beincluded in a Session Description File (SDF). The SDF file can bedownloaded by the client through the HyperText Transfer Protocol (HTTP)or Real-Time Streaming Protocol (RTSP) at the session start or announcedby the multicast server or a separate directory server (not shown inFIG. 1.) via the session announcement protocol (SAP) protocol.

In the present invention, each cell has a separate cell-dependentmulticast address for control message exchange (e.g. ARQ request andretransmission for program X) called herein an ARQ multicast address.The ARQ multicast address for a program X in a cell N is decoded fromthe source data multicast group address of program X and the mediaaccess control (MAC) address or the IP address of the AP/BS for cell N.If the MAC address of cell N AP/BS is MAC_N, the least significant mbits of the ARQ multicast address for a program X in a cell N is a hashfunction of the MAC address of cell N AP/BS, as shown in Equation 1.

{dxn(m−1), dxn(m−2), . . . dxn(0)}=Hash(MAC_N).   (1)

If Hash(MAC_N)=0, dxn(0) is set to be 1 in order to avoid the sameaddress as the source data multicast address of program X.

In an alternative embodiment, the least significant m bits of the ARQmulticast address for a program X in a cell N is a hash function of theIP address, IP_N, of cell N AP/BS, as shown in Equation 2.

{dxn(m−1), dxn(m−2), . . . dxn(0)}=Hash(IP_N).   (2)

If Hash(IP_N)=0, dxn(0) is set to be 1 in order to avoid the sameaddress as the source data multicast address of program X.

In another alternative embodiment, the least significant m bits of theARQ multicast address for a program X in a cell N is equal to the leastsignificant m bits of its AP/BS MAC address MAC_N or IP address IP_N.

The most significant (32-m) bits of the ARQ multicast address for aprogram X are equal to the most significant (32-m) bits of the sourcedata multicast address for the program X, i.e. dxn(31)=bx(31),dxn(30)=bx(30), . . . , dxn(m)=bx(m).

FIG. 3 is a flowchart depicting an exemplary method for a client toobtain the ARQ multicast address for a program X. A client associateswith an AP/BS at 305 in order to receive the multicastdata/content/program and obtains the MAC address or the IP address ofthe AP/BS at 310. The client then obtains the source data multicastgroup address of program X through configuration or session descriptionfile downloaded from a directory server, or an announcement from thedirectory server at 315. The client then decodes the ARQ multicastaddress of program X at 320 from the source data multicast address ofprogram X and the MAC address or the IP address of the AP/BS asdescribed above.

The ARQ multicast address of program X needs to be known by the ARQserver in order to receive the ARQ request from the client and tore-transmit FEC parity packets or source packets. The ARQ server may notknow the IP and/or MAC addresses of the APs/BSs and cannot decode theARQ multicast address from the IP and/or MAC address of the APs/BSs.Furthermore, the IP and/or MAC addresses of the APs/BSs may change, forexample, if a new AP/BS is added or if an existing AP/BS is removed oran existing AP/BS is replaced by another AP/BS with a different MACaddress, so that the ARQ multicast group of the ARQ multicast addressesused for a cell by the clients. Furthermore, the present inventiondescribes the method by which the ARQ server joins the ARQ multicastgroups when the ARQ multicast groups are needed and leaves the ARQmulticast groups after the ARQ multicast groups are terminated (this isan example of the MAC address of the AP/BS changing).

FIG. 4 is a flowchart illustrating an exemplary method for a client toregister with an ARQ server and send the ARQ server a request to receivepackets (FEC and/or source) from the ARQ server. At 405 the client sendsa request to join the source data multicast group and the ARQ multicastgroup by sending the request to the BS/AP or the Ethernet switch/router.The client's request to join the source data multicast group and the ARQmulticast group uses IGMP. The client then sends a registration messagein unicast to the ARQ server at 410. The registration message containsthe message ID, the address of the client, the address of the AP/BS withwhich the client is associated, the address and the port of the sourcedata multicast group, the address and the port of the ARQ multicastgroup. The client receives the source data of program X from the sourcedata multicast group at 415. If the client detects that some source datapackets are lost at 420 and the client does not detect an ARQ requestfrom other clients that requested the retransmission of FEC paritypackets and/or source packets that can be used to recover the lostsource packets at 425, the client may generate an ARQ request at 430.The client then performs a test at 435 to determine if the client hasreceived a registration reply from the ARQ server. The registrationreply message contains the address of the ARQ server and the address ofthe client. If the client has not received a registration reply, thenthe client performs a test at 440 to determine if the client haspreviously received retransmitted packets from the ARQ multicast group.If the client has not previously received retransmitted packets from theARQ multicast group then the client unicasts the ARQ request forretransmission of the packets (FEC and/or source packets) to the ARQserver at 445. The ARQ request is sent in unicast to the ARQ server ifthe client has not received the registration reply message from the ARQserver and if the client has not received any retransmitted packet fromthe ARQ server in the ARQ multicast group. Processing proceeds to 415.If the client has previously received retransmitted packets from the ARQmulticast group then the client multicasts the ARQ request forretransmission of the packets (FEC and/or source packets) to the ARQserver at 450. Processing proceeds to 415. If the client has received aregistration reply (at 435), then processing proceeds to 450. If theclient detected ARQ requests from other clients (at 425) for enoughpackets to recover the packets that the client lost the processingproceeds to 415. If the client did not detect any lost packets (at 420)then processing proceeds to 415.

The ARQ request message contains message ID, message type, round ID,source multicast address and port, ARQ multicast address and port, thesource coding block ID or the base sequence number (the sequence numberof the first source packet) of the source coding block, the number ofrequested retransmission parity packets, the average number of lostsource data packets in the source coding block, the length of the packetbitmap, the packet bitmap of the block, etc. The message type fieldindicates ARQ request for source data retransmission or FEC parityretransmission, or both source data and FEC parity retransmission. Theround ID indicates the number of the ARQ round during which the messagewas sent. For all receivers, the round ID of each FEC coding blockstarts with the value 0. The packet bitmap of the block indicates thestatus of received source packets in the source coding block, in which abit with value 1 denotes the corresponding source packet was correctlyreceived, and a bit 0 denotes the packet was lost. Note that the sourcemulticast address and ARQ multicast address above may be IP (layer 3)address and/or MAC (layer 2) address.

FIG. 5 is a flowchart depicting an exemplary method in accordance withthe principles of the present invention for an ARQ server to process aregistration message received from a client. The ARQ server receives aregistration message from a client at 505. As soon as the ARQ serverreceives a registration message from a client, a test is performed at510 to determine if the ARQ server has already joined the ARQ multicastgroup. If the ARQ server has not joined the ARQ multicast groupspecified in the registration message the ARQ server joins the ARQmulticast group specified in the registration message at 515. The ARQserver then sends a registration reply message in unicast to the clientafter it joins the ARQ multicast group at 520. If the ARQ server hasalready joined the ARQ multicast group specified in the registrationmessage then processing proceeds to 520.

FIG. 6 is a flowchart depicting an exemplary method in accordance withthe principles of the present invention for an ARQ server to process anARQ request message received from a client. The ARQ server receives anARQ request message from a client at 605. As soon as the ARQ serverreceives an ARQ request message from a client, a test is performed at610 to determine if the ARQ server has already joined the ARQ multicastgroup. If the ARQ server has not joined the ARQ multicast groupspecified in the ARQ request message the ARQ server joins the ARQmulticast group specified in the ARQ request message at 615. The ARQserver analyzes the ARQ request messages received from the clients in awireless cell at 620. Based on the ARQ request messages from the clientsin a cell, the ARQ server retransmits the FEC parity packets and/ororiginal source packets in the ARQ multicast group specified in the ARQrequest at 625. Note that the retransmitted FEC parity packets and/ororiginal source packets for different cells are retransmitted inmulticast in different ARQ multicast groups.

FIG. 7 is a flowchart depicting an exemplary method in accordance withthe principles of the present invention for an ARQ server to query anARQ multicast group. The ARQ server is notified that theARQ_MULTICAST_TIMEOUT has expired at 705. The expiration of theARQ_MULTICAST_TIMEOUT indicates that the ARQ server has joined the ARQmulticast group but has not received any ARQ requests from this ARQmulticast group for the period of the ARQ_MULTICAST_TIMEOUT. At 710, theARQ server sends a Multicast_Group_Query (MGQU) message to this ARQmulticast group. The MGQU message is used to query whether there is anyclient currently in this ARQ multicast group. The MGQU message containsthe address of ARQ server, the ARQ multicast address, the message ID,the message sequence number, etc. The ARQ server initializes theMulticast_Group_Query_retry_count at 715 and sets theMulticast_Group_Query_wait_timer at 720. TheMulticast_Group_Query_wait_timer is a timer, whose expiration time isMGQU_RETRIES_INTERVAL. After sending a MGQU, the ARQ server waits forthe Multicast_Group_Query_Reply (MGQR) message. A test is performed at725 to determine if the Multicast_Group_Query_Reply has been received.If the Multicast_Group_Query_Reply has not been received then a test isperformed at 730 to determine if the Multicast_Group_Query_wait_timerhas expired. If the Multicast_Group_Query_wait_timer has expired then atest is performed at 735 to determine if theMulticast_Group_Query_retry_count (MGQU_retry_count) is greater than apredetermined MULTICAST_GROUP_QUERY_RETRY_LIMIT (MGQU_RETRY_LIMIT). Ifthe Multicast_Group_Query_retry_count is less than or equal to theMGQU_RETRY_LIMIT, then the Multicast_Group_Query sequence number and theMulticast_Group_Query_retry_count are incremented at 740. TheMulticast_Group_Query is retransmitted at 745. Processing then proceedsto 720. If the Multicast_Group_Query_Reply has been received then theMulticast_Group_Query_wait_timer is cleared and theARQ_MULTICAST_TIMEOUT timer is reset at 750. If theMulticast_Group_Query_retry_count is greater than the MGQU_RETRY_LIMIT,then the ARQ leaves this ARQ multicast group at 755.

FIG. 8 is a flowchart illustrating an exemplary method in accordancewith the principles of the present invention for a client to process anARQ multicast group query from the ARQ server. The client receives anARQ multicast group query from the ARQ server at 805. A test isperformed at 810 to determine if the client is still a member of the ARQmulticast group specified in the ARQ multicast group query. If theclient is still a member of the multicast group specified in the ARQmulticast group query then the client sets theMulticast_Group_Query_Reply delay timer (MGQR_delay_timer) at 815. Atest is performed at 820 to determine if the client has received anyother Multicast_Group_Query_Replys from any other clients in the cell.If the client has not received any other Multicast_Group_Query_Replysfrom any other clients in the cell, then a test is performed at 825 todetermine if the Multicast_Group_Query_Reply_delay_timer has expired. Ifthe Multicast_Group_Query_Reply_delay_timer has expired then aMulticast_Group_Query_Reply is sent to the ARQ multicast group at 830.If the Multicast_Group_Query_Reply_delay_timer has not expired (at 825),then processing proceeds to 820. If the client has received otherMulticast_Group_Query_Replys from any other clients in the cell, thenthe client clears the Multicast_Group_Query_Reply_delay_timer at 835. Ifthe client is no longer (or has not been) a member of the ARQ multicastgroup specified in the ARQ multicast group query then the client ignorethe ARQ multicast group query.

The Multicast_Group_Query_Reply_delay_timer is a random delay timer,where Multicast_Group_Query_Reply_delay_timer(0<MGQR_delay_timer<MGQR_DELAY_LIMIT). This delay effectively randomizesthe time to send the Multicast_Group_Query_Reply (MGQR). The MGQR istransmitted to the ARQ multicast address from the client to all theparticipants (ARQ server and other clients in the ARQ multicast group).Within the delay period, a client may receive a MGQR from another clientin the same ARQ multicast group. If such a MGQR is received during thedelay period, this client will cancel its delay timer and will not sendits MGQR. MGQR contains the client address, the ARQ multicast groupaddress, the original MGQU message sequence number that this MRGRreplies to, etc.

The ARQ_MULTICAST_TIMEOUT, MGQU_RETRIES_INTERVAL, MGQU_RETRY_LIMIT, andMGQR_DELAY_LIMIT are the parameters that can be configured.

In an alternative embodiment, the client of a multicast program X sendsthe ARQ request in a cell independent control multicast group of themulticast program X if the client has not received the registrationreply message from the ARQ server and/or if the client has not receivedany retransmitted packets from the ARQ server in the cell dependent ARQmulticast group. The cell independent control multicast group uses acommon control channel.

The address of the cell independent control multicast group for themulticast program X, cx(31), cx(30), . . . cx(m+1), cx(m), cx(m−1), . .. cx0 can be assigned and configured at the multicast server, ARQserver, and clients. In an alternative method, the cell independentcontrol channel address for multicast program X can be contained in aSession Description File. The session description file can be downloadedby the client through the HTTP or RTSP protocol at the session start orannounced by the multicast server or a separate directory server. Inanother alternative method, the cell independent control channel addressof program X is deduced from the source multicast address of program X,for example, cx(31)=bx(31), cx(30)=bx(30), . . . cx(m+1)=bx(m+1),cx(m)=Abx(m), cx(m−1)=0, . . . cx0=0.

FIG. 9 is a block diagram of an exemplary client/receiving device inaccordance with the principles of the present invention. The controlmodule 905 is responsible for obtaining the source multicast address andARQ multicast address for a program. It is also responsible forregistering itself to the ARQ server, including sending a registrationmessage and processing the registration reply, receiving the multicastgroup query (MGQU) messages, processing MGQU messages, and sending themulticast group query reply (MGQR) via the communication interface 910.It informs the ARQ request module 915 of the address to send the ARQrequest, and informs the data receiving module 920 the addresses toreceive the source data and retransmitted data. The data receivingmodule 920 receives the source data and the retransmitted data via thecommunication interface 910. The data receiving module 920 informs theARQ request module 915 of any data loss. The data receiving module 920forwards the received data to the multicast application/multimediaplayer module 925. The ARQ request module 915 determines if an ARQrequest is necessary and what to request and sends the ARQ request tothe ARQ server via the communication interface 910. The ARQ requestmodule 915 also listens to the ARQ requests from other clients via thecommunication interface 910.

FIG. 10 is a block diagram of an exemplary recovery server/ARQ server inaccordance with the principles of the present invention. The controlmodule 1005 is responsible for client registration, including receivingand processing the client registration request, joining the multicastgroup and sending registration reply via the communications interface1010. The control module 1005 is also responsible for querying whetherthere are any clients using an ARQ multicast group, including sendingMGQUs, receiving MGQRs from the clients via the communication interface1010, and processing the MGQR messages and determining if the ARQ servershould leave this ARQ multicast group. The control module 1005 commandsthe data receiving/transmitting module 1015 to receive the data for aprogram (via the communications interface 1010) and stores the data in abuffer. The data receiving/transmitting module 1015 then receives thedata for a program (via the communications interface 1010) and storesthe data in the buffer. The control module 1005 advises the ARQ processmodule 1020 of the ARQ multicast address. The ARQ process module 1020receives the ARQ request via the communications interface 1010 andprocesses the ARQ request. The ARQ process module 1020 requests the datareceiving/transmitting module 1015 to retransmit the lost packets to theclient/receiving device. It is to be understood that the presentinvention may be implemented in various forms of hardware (e.g. ASICchip), software, firmware, special purpose processors, or a combinationthereof, for example, within a server, an intermediate device (such as awireless access point or a wireless router) or mobile device.Preferably, the present invention is implemented as a combination ofhardware and software. Moreover, the software is preferably implementedas an application program tangibly embodied on a program storage device.The application program may be uploaded to, and executed by, a machinecomprising any suitable architecture. Preferably, the machine isimplemented on a computer platform having hardware such as one or morecentral processing units (CPU), a random access memory (RAM), andinput/output (I/O) interface(s). The computer platform also includes anoperating system and microinstruction code. The various processes andfunctions described herein may either be part of the microinstructioncode or part of the application program (or a combination thereof),which is executed via the operating system. In addition, various otherperipheral devices may be connected to the computer platform such as anadditional data storage device and a printing device.

It is to be further understood that, because some of the constituentsystem components and method steps depicted in the accompanying figuresare preferably implemented in software, the actual connections betweenthe system components (or the process steps) may differ depending uponthe manner in which the present invention is programmed. Given theteachings herein, one of ordinary skill in the related art will be ableto contemplate these and similar implementations or configurations ofthe present invention.

1. A method, said method comprising: receiving a registration message;transmitting a reply to said registration message; receiving a recoveryrequest message; transmitting recovery data responsive to said recoveryrequest message; transmitting a message to a recovery multicast group todetermine status of said recovery multicast group; and receivingnotification that a recovery timeout has occurred.
 2. The methodaccording to claim 1, further comprising: determining if a recoverymulticast group specified in said registration message has already beenjoined; and joining said recovery multicast group if said recoverymulticast group has not already been joined.
 3. The method according toclaim 1, determining if a recovery multicast group specified in saidrecovery request message has already been joined; and joining saidrecovery multicast group if said recovery multicast group has notalready been joined.
 4. The method according to claim 3, wherein saidrecovery data is one of forward error correction packets and sourcedata.
 5. The method according to claim 3, wherein said recovery data isboth forward error correction packets and source data.
 6. An apparatus,comprising: means for receiving a registration message; means fortransmitting a reply to said registration message; means for receiving arecovery request message; means for transmitting recovery dataresponsive to said recovery request message; means for transmitting amessage to a recovery multicast group to determine status of saidrecovery multicast group; and receiving notification that a recoverytimeout has occurred.
 7. The apparatus according to claim 6, furthercomprising: means for determining if a device is a member of a recoverymulticast group; and means for transmitting a reply to said recoverydata, and wherein said transmitting act is performed if no other deviceshave replied to said recovery multicast group message and a delay timehas expired.
 8. The apparatus according to claim 6, further comprising:means for determining if a recovery multicast group specified in saidregistration message has already been joined; and means for joining saidrecovery multicast group if said recovery multicast group has notalready been joined.
 9. The apparatus according to claim 6, means fordetermining if a recovery multicast group specified in said recoveryrequest message has already been joined; and means for joining saidrecovery multicast group if said recovery multicast group has notalready been joined.
 10. The apparatus according to claim 9, whereinsaid recovery data is one of forward error correction packets and sourcedata.
 11. The apparatus according to claim 9, wherein said recovery datais both forward error correction packets and source data.